1. Field of the Invention
The present invention relates to a reproducing device for reproducing information stored in a recording medium such as an optical disk. The present invention further relates to a method of removing noise. Particularly, the present invention relates to a technology for canceling noise such as cross-modulation due to the adjacent track and crosstalk generated by the cross modulation.
2. Description of the Related Art
When an optical disk is used as a recording medium, high-density recording is achieved by decreasing the width of the track pitch of the optical disk.
However, in the case where the width of the track pitch is decreased, the width should be equal to or less than the diameter of a laser-light spot focused on the optical disk. That is to say, if the width of the track pitch is decreased without changing the diameter of the laser-light spot, signals of tracks adjacent to a main track would be reproduced together with signals of the main track. Therefore, the cross-modulation and the crosstalk are increased, thus deteriorating the S/N ratio, whereby recorded signals are incorrectly reproduced.
Here, “main track” as used herein means a track of a recording medium, such as a disk, that is currently being traced, that is, the track being irradiated with laser light by an optical pickup. “Adjacent tracks” as used herein means a track adjacent to the main track at the inner radius thereof and a track adjacent to the main track at the outer radius thereof.
In the past, crosstalk and cross-modulation have been reduced as described below.
An example where a reproducing device has an optical pickup that uses three light-beams including a main beam and two sub beams for reading signals is considered. In such a case, tracking control is performed so that the main track is irradiated with the main beam. Subsequently, reflection light is obtained and a signal corresponding to the main track is read out by using the reflection light. Further, the adjacent tracks are irradiated with the two sub beams. Then, based on outputs from reflection light obtained by the two sub beams, the values of crosstalk signals are estimated. By subtracting the estimated values of the crosstalk signals from the read main-track signal, crosstalk is cancelled. Thus, the crosstalk from the adjacent tracks, which disturbs the main-track signal to be reproduced, is decreased (corrected).
FIG. 5 is a block diagram illustrating a typical crosstalk-correction circuit that can be used for the above-described reproducing device.
It should be noted that a pickup including an optical system (not shown) irradiates an optical disk (not shown) with the main beam and the two sub beams.
The main track is irradiated with the main beam. The adjacent tracks on both sides of the main track are irradiated with the two sub beams.
Returned light obtained from the main beam is converted into an electrical signal by a light-receiving element 92. Subsequently, a main signal MX, which is a light-intensity signal corresponding to the main track, is obtained.
Returned light obtained from the sub beams is converted into two electrical signals by light-receiving elements 91 and 93. Subsequently, side signals S1 and S2, which are light-intensity signals corresponding to the adjacent tracks, are obtained.
By using the information about the adjacent tracks obtained by the light-receiving elements 91 and 93, the crosstalk from the adjacent tracks is corrected.
In the above-described case, the time difference between the three light beams, that is, the time difference between the side signal S1, the main signal MX, and the side signal S2, is corrected.
Then, the signals S1, MX, and S2 are each provided with a predetermined frequency characteristic by FIR filters 96, 97, and 98, respectively.
Next, the side signal S1 is added to the side signal S2 by an adder 99. That is to say, interference components from the two adjacent tracks are added together. Therefore, the signal output from the adder 99 becomes nearly equal to the crosstalk components from the two adjacent tracks.
Then, a multiplier 100 multiplies the signal output from the adder 99 by a predetermined constant α. Next, a subtractor 101 subtracts the multiplied result from the main signal MX. That is to say, the subtractor 101 cancels the crosstalk generated by the two adjacent tracks from the signal. Afterwards, the subtractor 101 outputs a corrected signal HX to be reproduced.
If a signal recorded on the main track is indicated by m(x) and signals recorded on the two adjacent tracks are indicated by s1(x) and s2(x), the above-described operations performed by the crosstalk-correction circuit shown in FIG. 5 can be indicated by Equation (1) shown below.H(x)=m(x)*psf1(x)−α{s1(x)+s2(x)}*psf2(x).  (1)
In Equation (1), the * symbol indicates convolution, α indicates the predetermined constant α to be multiplied by the signal output from the adder 99 by the multiplier 100, psf1(x) corresponds to the impulse response of the FIR filter 97, and psf2(x) corresponds to the impulse responses of the FIR filters 96 and 98.
Many crosstalk-correction circuits having the above-described configuration have already been proposed. Such crosstalk-correction circuits are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-167442, U.S. Pat. No. 6,442,114, U.S. Pat. No. 5,909,413, U.S. Pat. No. 5,729,514, and U.S. Pat. No. 5,544,141. Though the details of these crosstalk-correction circuits are different from that shown in FIG. 5, they all have substantially the same configuration as that of the crosstalk-correction circuit shown in FIG. 5. Further, the operations performed by these crosstalk-correction circuits are nearly equal to that shown by Equation (1).
However, in the cases where these crosstalk-correction circuits are used, the crosstalk effect changes according to the state of the main track. That is to say, the crosstalk effect changes according to whether or not there is a pit or a mark on the main track at the position where the laser-beam tracing is being performed. (If there is no pit nor mark on the main track at the above-described position, the surface at that position is called a mirror surface.) Therefore, the amount of cancelled crosstalk may become too large or too small, depending on the main-track state. That is to say, it is not possible to properly perform crosstalk canceling by using the above-described crosstalk-correction circuits.
The above-described problem will be described with reference to FIG. 6. When the spot of the main beam scanning the main track is positioned on an area A where the main track and the two adjacent tracks are all mirror surfaces M, the returning light intensity of the main beam is at Level LV1, which is the highest level. However, when the spot of the main beam is positioned on an area B where the main track is a mirror surface M, the adjacent track at the inner radius of the main track has a pit P, and the adjacent track at the outer radius of the main track is a mirror surface M, the returning light intensity of the main beam is at Level LV2, which is lower than the Level LV1, due to the effect of crosstalk generated by the pit of the adjacent track at the inner radius of the main track.
When the spot of the main beam is positioned on an area C where the main track is a mirror surface M, the adjacent track at the inner radius of the main track has a pit P, and the adjacent track at the outer radius of the main track has another pit P, the returning light intensity of the main beam is at level LV3, which is lower than the level LV2, due to the effect of crosstalk generated by the pits of the adjacent tracks at the inner radius and at the outer radius of the main track.
Thus, when the area of the main track irradiated with the main beam is a mirror surface M, the returning light intensity of the main beam is sharply decreased due to the effect of the pit P of the adjacent track as in the case of the area B. Particularly, in the case of the area C where each of the adjacent tracks at the inner radius and at the outer radius of the main track has the pit P, the returning light intensity is half as much as that of the case where there is no effect from the adjacent tracks, as in the case of the area A.
As shown in FIG. 6, when the spot of the main beam is positioned on an area D where the main track has a pit P and the adjacent tracks at the inner radius and at the outer radius of the main track are mirror surfaces M, the returning light intensity of the main beam is at level LV4. When the spot of the main beam is positioned on an area E where the main track has a pit P, the adjacent track at the inner radius of the main track has another pit P, and the adjacent track at the outer radius of the main track is a mirror M, the returning light intensity of the main beam is at level LV5, which is slightly lower than the level LV4, due to the effect of crosstalk generated by the pit P of the adjacent track at the inner radius of the main track. When the spot of the main beam is positioned on an area F where the main track has a pit P and each of the adjacent tracks at the inner radius and at the outer radius of the main track has another pit P, the returning light intensity of the main beam is at level LV6, which is slightly lower than the level LV5, due to the effect of crosstalk generated by the pits P of the adjacent tracks at the inner radius and at the outer radius of the main track.
Therefore, when the main track has a pit P at the position being irradiated with the main beam, as in the cases of the areas D, E, and F, the returning light intensity of the main beam is affected by the pit Ps on the adjacent tracks. However, the strength of the effect of the pit P from the adjacent track is smaller than in the cases of the areas A, B, and C where the main track is the mirror surface M.
Thus, the effect of the adjacent track is increased when the mirror surface of the main track is scanned. However, the effect of the adjacent track is decreased when the pit P of the main track is scanned. That is to say, the strength of the adjacent-track effect on the returning light intensity of the main beam changes depending on whether or not the main track has the pit P at the position that is being scanned by the main beam.
For solving the above-described problem, U.S. Pat. No. 6,084,837 discloses an apparatus for reproducing information recorded on a recording medium. The apparatus comprises a signal reader for outputting signals read from a main track and a track adjacent to the main track, a variable filter for filtering the signal read from the adjacent track and converting the signal into a crosstalk signal of the main track, a subtractor for subtracting the crosstalk signal from the signal read from the main track and a crosstalk-signal corrector for correcting the level of the crosstalk signal based on the signal read from the main track.
In the case where the above-described apparatus is used, the level of the crosstalk signal is decreased when the main track has a pit. However, the level of the crosstalk signal is increased when the main track is a mirror surface. In this manner, it becomes possible to properly adjust the amount of cancelled crosstalk for properly performing crosstalk canceling.
If a signal recorded on the main track is indicated by m(x) and signals recorded on two tracks adjacent to the main track are indicated by s1(x) and s2(x), the above-described operations performed by the crosstalk-signal corrector disclosed in U.S. Pat. No. 6,084,837 can be indicated by Equation (2) shown below, wherein α and β are constants:H(x)=m(x)*psf1(x)−{1+β·m(x)*psf1(x)}·{1+α·{s1(x)+s2(x)}*psf2(x)}.  (2)
A comparison of the above-described Equation (1) and Equation (2) is given below.
According to Equation (1), the information of the adjacent tracks is simply subtracted from the information of the main track. However, in Equation (2), the main-track information is multiplied by the adjacent track information. Such a component obtained by the above-described multiplication is generally referred to as cross-modulation.
If a signal recorded on the main track is indicated by m(x) and signals recorded on the two adjacent tracks are indicated by s1(x) and s2(x), the reproducing signal H(x), which is detected by an optical pickup, should be described as in the following equation:H(x)=m(x)*psf1(x)+α·{s1(x)+s2(x)}*psf2(x)+β·{s1(x)*psf3(x)}{m(x)*psf4(x)}+β·{s2(x)*psf3(x)}{m(x)*psf4(x)}.  (3)
In Equation (3), psf1(x), psf2(x), psf3(x), and psf4(x) are impulse responses, which differ from one another. Further, in this equation, the third and fourth terms (terms multiplied by constant β) indicate the effect of the cross-modulation.
Thus, in the case where an optical disk is used, two kinds of effect are caused by the adjacent tracks, namely, crosstalk components indicated by the term multiplied by constant α and cross-modulation components indicated by the terms multiplied by constant β. The frequency characteristic of the crosstalk components and that of the cross-modulation components are different from each other.
However, according to the related arts, the crosstalk generated by the adjacent tracks has been corrected or canceled by performing the approximate calculations shown in Equation (1) and Equation (2).
When the approximate calculation shown in Equation (1) is performed, the effect of the cross-modulation is ignored. Therefore, the noise cannot be properly corrected.
When the approximate calculation shown in Equation (2) is performed, the cross-modulation components shown in Equation (3) are not properly corrected.
Therefore, when the reproducing signal H(x), which is obtained according to the technique of related art, is decoded by maximum-likelihood decoding, an incorrect value may be obtained.